The present invention relates to a catalyst composition, to a process for its preparation and to its use in catalytic incineration of gases containing sulphur compounds, particularly hydrogen sulphide.
The presence of hydrogen sulphide (H.sub.2 S) in process waste gases, which are released into the air, is bound to very stringent rules in most industrialised parts of the world. Waste gases containing H.sub.2 S are produced in many different processes, such as, for instance, in the well-known Claus process. This Claus process produces an effluent which may contain over 5% by volume of H.sub.2 S in addition to other sulphur compounds. A known method for reducing the level of sulphur compounds and H.sub.2 S in the Claus off-gas is, for instance, subjecting this off-gas to a catalytic reduction treatment, thereby converting the sulphur compounds present into H.sub.2 S, and subsequently removing the bulk of H.sub.2 S by an absorption treatment using a suitable H.sub.2 S-selective absorption solvent The absorption solvent containing the bulk of the H.sub.2 S is then regenerated, after which the desorbed H.sub.2 S is returned to the Claus-unit and the cleaned solvent is re-used. The final off-gas from the absorption treatment containing only minor amounts of H.sub.2 S is normally incinerated, thereby converting H.sub.2 S into sulphur dioxide (SO.sub.2), which is less harmfil than H.sub.2 S. Tolerable levels of SO.sub.2 in waste gases released into the air are, consequently, much higher under air pollution regulations than the tolerable levels of H.sub.2 S. The incinerated gas should normally contain less than 10 ppm on a volume basis of H.sub.2 S.
Incineration of H.sub.2 S-containing waste gases nowadays is normally conducted either in a catalytic process or in a non-catalytic (thermal) process. Major considerations for applying a catalytic incineration process are, relative to a non-catalytic incineration process, a reduced heat input and a more selective formation of SO.sub.2, whereby the formation of SO.sub.3 is suppressed as much as possible for reasons of corrosion and air pollution abatement.
In U.K. patent specification No. 1,558,656 a process for the catalytic incineration of H.sub.2 S-containing waste gases is disclosed, wherein such waste gas is contacted with a stoichiometric excess of oxygen having regard to the contained H.sub.2 S in the presence of a catalyst composition comprising copper (Cu) and bismuth (Bi) as the catalytically active components supported on a carrier material, which suitably is alumina. Although the CuBi/alumina performs satisfactorily in many respects, there is still room for improvement, particularly in terms of reducing the activity of the catalyst for H.sub.2 oxidation and for the undesired formation of the corrosive SO.sub.3 and H.sub.2 SO.sub.4. The occurrence of H.sub.2 oxidation is undesired, because the heat generated in this exothermic reaction may lead to a thermal run away reaction. By reducing the activity of the incineration catalyst for H.sub.2 oxidation, the risk of a thermal run away reaction is reduced, which obviously is beneficial for reasons of process control. Furthermore, the H.sub.2 oxidation may also trigger other undesired reactions, such as the conversion of CO into COS and the (thermal) conversion of hydrocarbons. Reducing the formation of SO.sub.3 and H.sub.2 SO.sub.4 by increasing the selectivity of the catalyst for the conversion of sulphur compounds, in particular H.sub.2 S, into SO.sub.2 is also desired for environmental reasons (reduced air pollution) and for reasons of a reduced corrosion of the equipment used.
It has been found that a catalyst comprising at least one Group IIA metal in addition to bismuth, chromium or molybdenum indeed reduces H.sub.2 oxidation and formation of SO.sub.3 and H.sub.2 SO.sub.4, whilst at the same effectively converting H.sub.2 S and other gaseous sulphur components that may be present in a waste gas or off-gas, such as carbonyl sulphide (COS) and carbon sulphide (CS.sub.2), with oxygen into SO.sub.2.